1. Field off the Invention
This invention relates to a numerical control unit for a non-circular workpiece fabricating machine, and more particularly to a numerical control unit which controls the rotation of a main spindle for rotating the workpieces and displacement of tools for fabricating the workpieces in such a machine.
Further, the invention relates to a numerical control unit for a numerically controlled grinding machine (called "NC grinding machine" hereinafter) to fabricate non-circular workpieces, and controls the rotation of the main spindle and displacement of a wheel spindle stock. Specifically, the numerical control unit automatically produces data denoting a new and smooth shape of the workpiece (called "new lift data") by eliminating errors from existing data on the shape of a non-circular workpiece. Using the new lift data, the control unit then produces X/C-axis data which denote displacement position of the wheel spindle stock and a rotation angle of the main spindle on the basis of a diameter of a grinding wheel and period of time per rotation of the workpiece. The rotation of the main spindle and displacement of the wheel spindle stock are controlled on the basis of the X/C-axis data.
2. Description of the Related Art
Generally, an NC grinding machine is used to fabricate non-circular workpieces such as cams by synchronously controlling a main spindle and displacement of a wheel spindle stock. X/C-axis data which denote displacement of the wheel spindle stock and a rotation angle of the main spindle (i.e., pitch) are usually prepared beforehand based on data (lift data) representing the shape of the non-circular workpiece, a diameter of a grinding wheel and period of time per rotation of the workpiece. When designing a cam, theoretical values are usually determined-first, and then a reference model is fabricated, modified and measured to meet the requirements of an actual cam. Data on the reference model are used to obtain lift data to actually fabricate cams. Such data usually contain errors during the designing, modification and measurement. When the workpiece is fabricated according to X/C-axis data prepared based on such lift data, either the main spindle or the wheel spindle stock tend to vibrate, thereby causing the workpiece to have a rough surface and be poor in its performance. Further, a wheel spindle stock displacing mechanism is prone to a problem such as repeated grinding stress, which disadvantageously shortens the life of the NC grinding machine.
Therefore, the lift data are usually smoothed and interpolated so as to eliminate the errors. The smoothing and interpolation are carried out by a non-circular data pre-processing unit 101 as shown in FIG. 10 of the accompanying drawings. In the non-circular data pre-processing unit 101, a smoothing/interpolation calculating section 102 smoothes and interpolates the lift data, which are stored in a lift data memory 103, based on a spline function. The smoothed and interpolated data (called "new lift data") are stored in a new lift data memory 104. Next, an X/C-axis data calculating section 107 produces X/C-axis data based on the new lift data, data on the diameter off the grinding wheel (stored in a grinding wheel data memory 105), and rotation speed data of the main rotary spindle (stored in a main spindle rotation speed memory 106). The obtained X/C-axis data are stored in an X/C axis data memory 108.
The NC grinding machine fabricates the non-circular workpiece based on the X/C-axis data which have been smoothed and interpolated.
The foregoing smoothing and interpolation are usually carried out only at each predetermined interval as shown in FIG. 11. In other words, the smoothing and interpolation at the constant and predetermined interval cannot eliminate the errors sufficiently, and cannot assure a smooth smoothing/interpolating curve. Therefore, if the workpiece is fabricated based on the X/C-axis data which are prepared according to the foregoing new lift data, the wheel spindle stock tends to irregularly accelerate in the moving direction thereof as shown in FIG. 12. Such irregular acceleration leads to vibrations of the wheel spindle stock and the grinding wheel, which would lower the quality of the fabricated workpiece and shorten the effective life of the grinding machine.